Positioning systems which provide attitude information are utilized in a variety of applications. For example, backpacks, planes, missiles, boats, helicopters, and vehicles often require information as to their attitude factor, e.g. the direction the vehicle is pointing in a horizontal plane (e.g., the azimuth angle) and the direction the vehicle is pointing in a vertical direction (e.g., the elevation angle). Alternatively, other coordinate systems can be utilized to provide attitude information in a variety of forms and coordinates.
Heretofore, attitude information has been generated by mechanical and electrical devices. Mechanical devices that include levels and compasses can provide indications of azimuth and elevation angles. However, levels and compasses are prone to wear and are somewhat inaccurate.
Electrical attitude determination systems often are part of expensive inertial navigational systems or satellite positioning systems, such as, global positioning systems (GPS). Satellite positioning systems, such as, global positioning system (GPS) navigational systems, are often used by military and civilian naval, ground, and airborne vehicles for navigation. GPS receiver units receive positioning signals from a series of 24 Navstar satellites deployed in 12-hour orbits about Earth and dispersed in six orbital planes at an altitude of 11,200 miles. The satellites continuously emit electronic GPS signals (or telemetry) for reception by ground, airborne, or naval receiver units. By receiving GPS signals from four or more satellites, a properly configured receiver unit can accurately determine its position in three dimensions and time.
GPS navigational systems have tremendous benefits over other positioning systems in that they rely upon no visual, magnetic, or other point of reference. These advantages are particularly important in applications, such as, aviation, naval navigation, and defense applications where polar regions are traversed and where conventional magnetic navigational devices are rendered less effective by local magnetic conditions.
Conventional GPS navigational systems capable of attitude determination, such as, system 10 illustrated in FIG. 1, generally include at least three antennas 12A-C coupled to at least three GPS receiving units 14A-C and a processor 16. A receiving unit is defined as a device that processes one signal input of available Code Division Multiple Access (CDMA) satellite signals with one or more demodulation channels that are each capable of outputting the code and/or carrier phase information from a selected satellite signal. System 10 determines a coordinate position for the vehicle (not shown) upon which it is implemented based upon satellite signal code and carrier phase information received on buses 18A-C from receivers 14A-C. Additionally, processor 16 compares satellite signal code and carrier phase information to compute attitude information at an output 28 for system 10. System 10 can compute the attitude information in a variety of coordinate systems for display or further use.
Generally, kinematic techniques use the comparison of carrier phase information from different satellites to determine the relative positioning of antennas 12A-C. Processor 16 utilizes the relative positioning of antennas 12A-C to determine the attitude information. The calculation of the attitude information from the carrier phase information on buses 18A-C is well known in the art and is beyond the scope of the present application.
To make accurate comparisons of the code and the carrier phase information on buses 18A-C, receivers 14A-C must be synchronized via a synchronization bus or signal 22. Receivers 14A-C must be synchronized to ensure that time, frequency, and phase references associated with the satellite signals received on antennas 12A-C are accurate. Synchronization can be very difficult and adds to the cost and complexity associated with system 10. Additionally, system 10 requires at least three separate GPS receivers 14A-C (one for each antenna), which can be expensive and can add to the size, power consumption, and complexity of the system 10.
Thus, there is a need for a satellite navigational system which can determine attitude information without the use of multiple receivers. Further still, there is a need for a GPS navigational system which does not require synchronization between individual receiver units. Even further still, there is a need for a low-cost GPS navigational system capable of attitude determination.